The present invention relates to an airbag apparatus mounted in a vehicle to protect an occupant of the vehicle or pedestrians from an impact and, more particularly, to an airbag apparatus having an airbag including a vent hole and a gas releasing valve, which selectively opens and closes the vent hole.
Airbag apparatuses are widely known as apparatuses for protecting occupants of vehicles or pedestrians. An airbag apparatus has an airbag, which is formed by sewing peripheral portions of a pair of fabric sheets together, and an inflator arranged in the airbag.
In one such air bag apparatus, such as a side airbag apparatus for protecting an occupant from an impact from the side, the airbag is incorporated in a seat back of a vehicle together with the inflator.
When an impact is applied to a body side portion of the vehicle from the side, the side airbag apparatus supplies inflation gas from the inflator into the airbag. The inflation gas inflates and deploys the airbag, and the airbag comes out of the seat of the vehicle with a part of the airbag remaining in the seat back. The airbag is inflated and developed forward in the narrow space between the occupant seated on the seat and the body side portion of the vehicle. When inflated and developed, the airbag is arranged between the occupant and the body side portion, which projects into the interior of the vehicle, restrains and protects the occupant. After the airbag is inflated and developed, the excessive inflation gas in the airbag is discharged to the outside of the airbag through a vent hole formed in the airbag.
As a technique related to such vent holes, Japanese Laid-Open Patent Publication No. 2007-22306, for example, discloses an airbag apparatus having a gas releasing valve that opens a vent hole according to the pressure in the airbag. The airbag apparatus employs a sheet having an opening formed at the center as a gas releasing valve. The gas releasing valve has a first sheet portion and a second sheet portion, each of which forms a corresponding half of the sheet with respect to the opening, which is the boundary. The gas releasing valve is arranged outside the airbag in such a manner that the opening corresponds to the vent hole. The gas releasing valve is then joined to the airbag along the periphery of the vent hole. Further, the first sheet portion and the second sheet portion are joined together in such a manner as to close the opening of the sheet. When the pressure in the airbag becomes greater than or equal to a predetermined value, the first sheet portion and the second sheet portion of the gas releasing valve are separated from each other, thus exposing the vent hole.
The airbag apparatus maintains the vent hole closed by the gas releasing valve until the airbag is inflated and the pressure in the airbag reaches the predetermined value. This prevents the inflation gas from escaping to the outside of the airbag through the vent hole. This quickly raises the pressure in the airbag, thus promoting, or in other words, increasing the speed of inflation of the airbag. When the pressure in the airbag becomes greater than or equal to the predetermined value, the first sheet portion and the second sheet portion of the gas releasing valve are disconnected from each other at the joint portion, in such a manner as to expose the vent hole. This discharges the inflation gas from the vent hole to the outside of the airbag, thus decreasing the pressure in the airbag. As a result, the occupant is protected from an impact by the airbag having an appropriate level of hardness.
In the airbag apparatus described in the aforementioned document, the vent hole is opened by operating the gas releasing valve according to the pressure in the airbag. However, the vent hole is opened by rupturing the joint portion between the first sheet portion and the second sheet portion of the gas releasing valve. Such a method, which physically breaks the joint portion, makes it difficult to open the vent hole in response to a certain level of pressure in the airbag. In other words, it is difficult to suppress variation of the rupture strength of the first sheet portion and the second sheet portion of the gas releasing valve. This varies the timings at which the vent hole is opened. Accordingly, the airbag apparatus must be improved in order to suppress such variation of the valve opening timings and enhance the stability for opening the gas releasing valve.